Nucleic acid hybridization studies using cloned retroviral DNA were used to examine the evolution and organization of primate and carnivore retroviruses within the mammalian genome. Among the Felidae, only six small species of cats have been found to contain nucleic acid sequences related to RD-114 and FeLV viruses. Studies with subgenomic probes from the endogenous RD-114 virus have shown conservation among fragments in the gag and pol viral regions of domestic cat DNA. These same fragments are also found in multiple copies in cell DNA from the other five RD-114-containing cat species. The leopard cat from Southeast Asia, Felis bengalensis, does not contain either RD-114 or FeLV viral sequences in its DNA. The offspring of matings between the leopard and domestic cats thus afford a unique opportunity to study the segregation of both sets of virogenes in the progeny of backcrosses of F1 hybrids to the virogene-negative parent. Liquid hybridization studies with radioactively labeled DNA probes prepared from RD-114 and FeLV viruses reveal that F1 hybrids contain approximately one-half of the number of virogene copies present in the domestic cat parent. When restriction digests of cell DNAs from backcross offspring were analyzed with probes specific for RD-114, different individuals exhibited different combinations of viral fragments. Analogous results have been obtained with subgenomic FeLV probes. These results demonstrate that viral sequences from both endogenous families are dispersed on multiple chromosomes throughout the cat genome. Segregation of different RD-114 sequences in cat x rodent somatic cell hybrids has confirmed this conclusion. Genetic mapping of sequences from both families of viruses in the backcross animals will be accomplished by correlating the occurrence of unique viral restriction fragments with feline isozymes previously assigned to specific cat chromosomes. A phylogenetic tree of carnivore evolution has also been derived by molecular hybridization techniques - it differs in many respects from the classical relationships derived by anatomical considerations and morphometric measurements.